1. Field of the Invention
This invention generally relates to secure transmission of digital voice and data communications. More particularly, the invention relates to a stream cipher with a plurality of linear feedback shift registers generating large pseudo-random bit sequences and having multiple security keys.
2. Description of the Prior Art
Code division multiple access (CDMA) is a type of spread-spectrum communication system wherein each subscriber unit is distinguished from all other subscriber units by the possession of a unique code. In order to communicate with a particular subscriber unit, a transmitter unit imprints the unique code upon transmission and the receiver uses the same code to decode the transmission.
The unique codes used by a CDMA communication system to transmit voice and data communications appear noise-like and random. Since the random sequences are generated by standard deterministic logic elements, the generation of the bit sequences are predictable and repeatable. It is the use of these repeatable binary random sequences that permits easy modulation with any information-bearing signal. These predictable random sequences are called pseudo-random sequences.
Each transmitter within a CDMA communication system includes a stream cipher generator which uses a key to encipher the voice and data communications. An identical stream cipher generator at the receiver deciphers the received enciphered communications using the same key.
As is well known in the prior art, the simplest stream cipher generator is the linear feedback shift register. A shift register of a finite bit length is clocked at a fixed rate. An exclusive-OR (XOR) gate generates the serial input signal from the XOR combination of some bits of the shift register. The circuit then proceeds through a set of states, eventually repeating itself after a finite number of clock pulses. However, the stream cipher generated by linear feedback shift register is related to the length of the shift register and which bits are combined in the XOR to generate the next input. If a complex stream cipher is desired, an expensive shift register having a cumbersome length must be used.
Accordingly, there is a need for a simple method of increasing the complexity of stream ciphers to increase security of enciphered messages.
One embodiment of the invention is a subscriber unit for transmitting communication signals. The subscriber unit comprises a cipher stream generator which generates a cipher stream to encipher a digital data stream. A data stream mixer mixes the cipher stream with the digital data stream. An antenna radiates the mixed cipher and data stream as a communication signal.
Another embodiment of the invention is a subscriber unit for receiving communication signals. The subscriber unit comprises a cipher stream generator for generating a cipher stream with the received communication signal to produce a decoded data stream.
The cipher steam generator includes first and second linear feedback shift registers. Each has a clock input and an output. The outputs are combined to generate the cipher stream. The output of the second register is combined with a clock signal which is inputted to the clock input to the first register.